The invention relates to an oil pump unit including a plurality of pumps and a hydraulic fluid supply arrangement for selectively supplying hydraulic fluid discharged from these pumps to a hydraulic apparatus.
Considering a power steering system, for example, which is mounted on an automobile to reduce the magnitude of the force which is required for a driver to operate a steering wheel, an oil pump may be used as a source of oil pressure. Such oil pump is driven for rotation by the engine of the automobile, and has a discharge which increases or decreases in proportion to the number of revolutions of the engine. Accordingly, it is necessary that such pump has a sufficient capacity to supply enough fluid to operate the hydraulic apparatus such as the power steering device properly even if the engine operates at a low number of revolutions or with a reduced discharge from the pump.
However, it will be seen that if the pump is provided with such capacity, it follows that an unnecessarily large amount of fluid is supplied when the engine operates at a higher number of revolutions. This not only results in a waste in itself, but also increases the dissipation of the horsepower of the engine which is used to drive the pump to cause a great influence upon the fuel cost of the automobile engine, which is undesirable for the purpose of power saving.
To cope with this problem, there has been provided an arrangement in the prior art which includes a combination of a pair of pumps, each of a reduced capacity, and a control having a flow path switching function so that whenever the discharge from the respective pumps is small, their flow is combined while the hydraulic oil from one of the pumps only is supplied to the power steering device if the discharge from each of these pumps increases, with the other pump being connected to a tank to return the hydraulic oil therefrom. In this manner, the horsepower required to drive the other pump is minimized in order to reduce the dissipation of the horsepower.
The described arrangement is constructed such that a switching of the flow path takes place in accordance with the discharge from each pump or the number of revolutions of the engine. Accordingly, while the dissipation of the horsepower can be reduced when the automobile is running at a high speed or the engine is operating at a higher number of revolutions, a power loss is unavoidable at a lower number of revolutions of the engine, leaving much to be improved.
It is to be noted that the supply of hydraulic oil to the power steering device presents a problem when a high outout is demanded therefrom in response to a high load, or during a steering operation. At other times, for example, when the vehicle is at rest or running straightforward, the supply of hydraulic flow may be maintained low if the engine is operating at a low number of revolutions. In particular, mode running patterns, for example, are frequently utilized with automobiles which run through the city, and hence it is desirable that the dissipation of the horsepower be reduced when the vehicle is running at such low speeds.
This objective can be achieved by employing a control including a flow path switching mechanism which operates by sensing an increased load upon the power steering system. However, a problem arises with such arrangement in that a switching of the flow path takes place to cause an increased dissipation of the horsepower if the engine is operating at a high speed and hence the discharge from a single pump is sufficient to meet the need.
Alternatively, an arrangement is also proposed in which the running speed of an automobile is detected electrically, and a detection signal is utilized to effect a switching of the flow path. However, the vehicle speed is not always proportional to the number of revolutions of the engine or the discharge from the pump, and accordingly, this arrangement cannot assure an effective reduction in the dissipation of the horsepower. In particular, such problem occurs with an overloaded truck which may be running at a low speed while the engine is operating at a higher speed. In addition, the use of electrical detection means and associated solenoid valves involve problems relating to the construction.